The invention relates to an optical element having a region that is arranged on a first substrate and that comprises at least one layer, said region containing a reactive material, a dye, and ion storage, whereby in said ion storage are present at least one redox system and at least one ion that can penetrate into said reactive material.
A number of optical elements exist that reversibly change their optical or electrical properties by means of illumination or external wiring. Electrochromic cells and electrochromic dye-sensitive solar cells are cited as examples.
An electrochromic cell having two electrodes and an electrolyte that is arranged between said electrodes and that contains dissociated LiI is known from xe2x80x9cPhotoelectrochromic Windows and Displays,xe2x80x9d C. Bechinger et al., Nature, Vol. 383, Oct. 17, 1996, pp. 608-610. One of the two electrodes comprises glass that is coated with indium-tin oxide (ITO), upon which electrochromic WO3 is applied as a reactive material. The other electrode comprises nanocrystalline TiO2, upon which a dye layer is arranged.
The absorption of incident light by the dye causes an injection of electrons into the TiO2 layer, which functions as a charge carrier collecting layer. The oxidized dye is then reduced again by electron transfer from the iodine ions. As a result of this process, a potential difference forms between the two electrodes. If said electrodes are now electrically connected by an external circuit, the electrons injected into the TiO2 flow into the colorless WO3 layer, and at that point lead to reduction and a resultant coloration of said WO3. Simultaneously with the electrons, lithium ions migrate from the electrolyte into said WO3 to maintain charge neutrality. In contrast to electrochromic cells, said element for colorization of the WO3 layer advantageously requires no external voltage source.
An equilibrium state arises in this element as a function of the intensity of illumination. If the illumination is reduced by closing the circuit, the electrons flow back into the electrolyte and the coloration disappears. On the other hand, when the circuit is opened, the illumination is diminished, and the electrons remain in the WO3 layer and the color is maintained for several hours.
However, decolorization for this electrochromic element is possible only in the unilluminated state. The reaction that leads to decolorization in the unilluminated state necessarily occurs also during colorization in the illuminated state. The dye reaction is thus slower, and the colorization that arises in the equilibrium state is less than would be the case without the parallel decolorization reaction. A desired rapid decolorization can be achieved only with simultaneous reduction of the rate and depth of the dyeing. A further disadvantage is that external wiring cannot be dispensed with, and the function of such optical elements necessarily requires the presence of two electrodes.
Proceeding from the disadvantages of the prior art, the object of the invention is to provide an optical element that allows a more rapid and intensive optically-induced alteration of the optical or electrical parameters of a stratified region, and that additionally can be designed more simply than comparable optical elements of the prior art.
This object is preferably achieved by the characterizing features of the invention. Advantageous embodiments and further developments of the invention will be apparent from the description of the invention provided herein.
An optical element having a substrate and a region that is arranged thereon and that comprises at least one layer, said region containing a reactive material, a light-sensitive material (hereinafter called a dye), and ion storage, allows rapid and intense alteration of the properties of said reactive material, provided that said dye is in contact with said reactive material, either directly or separated by no more than one electrically conductive layer. This development of said optical element assures direct charge transfer between said dye and said reactive layer within the stratified region, and allows the electrically conductive electrodes and wiring (FIG. 1), as instructed by prior art, to be dispensed with. By means of an advantageous spatial separation of the colorization and decolorization reactions, a switchable development of the invention (FIG. 2) avoids the simultaneous appearance of said colorization and decolorization reactions that occur with traditional electrochromic optical elements. In this manner, the colorization is, in principle, more rapid and more intensified. Furthermore, this development, in contrast to the conventional elements, allows switchable decolorization in the illuminated state.
With each charge carrier that is injected from the dye into the reactive material, an oppositely charged ion also migrates from ion storage into said reactive material to compensate for said charge carrier. In the region of the ion storage containing a redox system, a redox reaction therefore takes place to maintain the charge neutrality of the dye. Although the optical element according to the invention can be more simply designed compared to optical elements of prior art; since, according to a feature of the invention, semiconductors, electrodes, and an external circuit in particular can be dispensed with, the optically induced alteration of the optical or electrical parameters of said reactive material according to the invention is advantageously achieved more quickly.
Suitable as reactive materials are those substances whose optical or electrical properties, such as conductivity, change as a result of the charge carriers injected from the dye. Thus, for example, electrochromic materials can be used whose color, and optionally, electrical properties, changes due to an induced alteration of the oxidative state caused by the injected charge carriers. Substances suitable for dyes include those that are used in the area of dye-sensitive solar cells. Said dye and reactive material can be arranged either in layers atop one another or together as a single layer. The use of porous layers, such as those produced from the solar get method, of reactive materials is thus conceivable, whereby said dye is embedded in the pores of said reactive material. Intermixture of said reactive material and said dye leads to further improvement of the intensity and speed of the optically induced alteration of the parameters of said reactive material.
Substances suitable as ion storage are those that can accept ions and also display at least slight ion conductivity. The requirement for ion conductivity of said ion memory, however, is significantly less than for electrochromic dye-sensitive solar cells, for example, since it is usually not necessary to maintain an ionic current between two electrodes. It is only in the decolorization process in the illuminated state, for a switchable development (FIG. 2), that an ionic current must flow through the entire ion storage, from the left to the right electrode. Otherwise, a local reaction of said ion storage directly at the interface, for example, is sufficient.
The ion storage contains at least one ion that can penetrate into the reactive material as compensation for the charge carriers injected from the dye, and also contains at least one redox system that maintains the charge neutrality of said dye and, optionally, of said ion storage. Said ion storage can likewise have a stratified design, and can be arranged, for example, on a dye layer applied to said reactive material or on a layer containing said dye and said reactive material as a mixture. However, it is also possible to have a mixture of reactive material, dye, and ion-storing substance in a single layer. Thus, the interstices of a nanocrystalline reactive material can be filled with a liquid ion-storing electrolyte and said dye.
Other substances such as catalysts or charge carrier-collecting materials can also be incorporated into a layer of the region according to the invention that contains at least one layer, or as an additional layer in said region. Such materials promote, for example, the process kinetics or stability of the system. The material that collects charge carriers is preferably arranged between the dye and the reactive material, and improves the charge transfer efficiency of said dye in said reactive material, thereby improving the process kinetics.
For an element according to the invention, illumination creates an equilibrium state between, on the one hand, an alteration of the optical or electrical properties, or both of said properties, of the reactive material induced in same by the injection of charge carriers (forward reaction), and on the other hand, a simultaneous reverse reaction. Said reverse reaction (charge carrier return flow) can take place via the same interface, between the dye and said reactive material, as for the forward reaction, or alternatively, via external wiring and a second electrode that is arranged on the side of the ion storage facing opposite said reactive material and that is designed as a substrate or layer. Said external wiring can contain any desired switchable element.
The equilibrium state under illumination is known as an intercalated state, since said state can be characterized, among other criteria, by the number of compensating ions from ion storage that have penetrated (intercalated, for example) into the reactive material. If the illumination is reduced, the reverse reaction dominates the forward reactionxe2x80x94although said reverse reaction can be strongly kinetically inhibitedxe2x80x94until an equilibrium state has again developed at a lower level (deintercalated state). If the reverse reaction kinetics are kept at a slow rate, the intercalated state can be maintained for several hours, even in the dark, and the optical element functions as storage. If this storage effect is not desired, the reverse reaction kinetics can be accelerated by the addition of a catalyst, for example. External wiring of the optical element according to the invention can also influence said reverse reaction.